1. Field of the Invention
This invention relates to a prober and tester for testing electrical characteristics of integrated circuits (IC's) which have been fabricated on silicon or other semiconductor wafers or other substrates to determine which IC's are operational. More particularly, the invention is directed to an apparatus which permits testing of a wafer in vertical orientation where a prober can be docked to a tester with required precision and with simplicity.
2. Related Art
Integrated circuits (IC's) are fabricated on silicon wafers and are routinely tested on the wafer to determine which IC's are operational. After testing, the wafer is broken up into individual dies and each operational working die(s) is mounted into a ceramic, plastic or other package with external leads that can be connected to other devices. The operational objective for testing on the wafer, for the IC manufacturers, is to only pay for packaging IC's which are operational i.e. have passed the testing. During testing, IC's which "FAIL" are suitably marked and then discarded prior to packaging of IC's which have "PASSED".
Testing has become progressively more challenging because as individual device geometry in the integrated circuit shrink, the frequency of electrical signals in the device, commonly called clock speed, of the devices increases roughly in proportion. Further, testing at the wafer probes must mimic the final application as closely as possible. As a result, the test equipment also must operate at progressively higher frequencies.
In most recent wafer prober prior art, a test head of a tester is mounted on a head plate of the wafer prober, as typically seen in U.S. Pat. No. 4,517,512. An insert ring is mounted in the head plate. A contact board is disposed on an upper surface of the insert ring with a card socket on the bottom surface. These are connected by cabling and a probe card is attached to the card socket by connector pins or sockets. A test head is positioned over the head plate with a performance board therebetween. The performance board and contact board are connected by connector pins. A chuck with an IC's-containing wafer mounted thereon is positioned below the probe card and needle-like contacts from the probe card contact predetermined bonding pads on each IC.
A major problem in mimicking the final application in test is that, in the final application, the packaged IC's are mounted a few millimeters apart connected by fine copper lines on a printed circuit board. In test, the connection to the IC must be made from a test computer that generates the logic test patterns, to D/A (digital-to-analog) converters that convert the logic signals to an electrical signal that varies in time, through a set of interconnections, typically cables, to a probe card, and finally to the IC. In traditional test configurations, a cable extends from the probe card positioned above a wafer to a tester cabinet containing the D/A converters. In U.S. Pat. No. 4,517,512 the D/A converters are mounted on the probe card of a test head and cabling extends from D/A converters to the tester. The probe card comprises a set of probe needles arranged to make contact with the pads on the integrated circuit. The system is designed in this way so that specific test conditions can be generated on the host computer under software control. The tester must exercise the chip through as many sets of conditions as possible.
The actual testing process uses both a tester and a prober. A prober, as made by Electroglas Inc., Santa Clara, Calif. as model number 4080 or 4085, is a machine that moves the wafer around underneath a probe card, and probes ICs on a horizontally oriented wafer by making repetitive contacts between numbers of small pads on each IC and the probe needles. A similar system is seen in U.S. Pat. No. 5,172,053. The base of the prober contains electronics, a ring carrier consisting of a metal plate with a hole for a probe card, the ring carrier being mounted to the base by screws into a series of posts. The probe card is mounted in the hole in the ring carrier. The wafer is held on an XY stage chuck more particularly on the chuck top via a set of vacuum rings. The chuck top is mounted, using screws to a Z/theta stage, that can move vertically and in rotation using stepper motors coupled to lead screws as seen in U.S. Pat. No. 4,066,943. The Z stage also has vacuum pins that are used to transfer wafers as will be described later. The Z/theta stage is mounted to an XY stage, and the Z/theta stage is used to force the probe needles into contact with the pads on each successive IC. Every IC on the wafer can be tested. As used by Electroglas, the Z/theta stage can be a ball bearing assembly as described in U.S. Pat. No. 5,344,238 and sold as Electroglas Model #PZ7, and the XY motor can be a linear stepper motor as described in U.S. Pat. No. 3,940,676. There is often a robot added to the prober to automatically remove wafers from a cassette and load them on the chuck top as in Electroglas model 4080 and 4085.
The earliest tester connections in prior devices were made from D/A converters buried in the tester, through a series of cables running from the tester to the prober. As the test frequencies became higher the length of the cables from the D/A converters and the probe card limited the maximum test frequency. One solution as taught in U.S. Pat. No. 4,517,512 was to place the D/A converters in a test head directly over the prober and then make a very short cable connections to the probe card. These cables carry less current and therefore allow higher frequency testing than the original cables to the probe card shown in prior devices.
There are a number of practical constraints that affect the length of cable and type of connections that can be used.
1) The tester must be able to connect to probers as described above and other types of machines that handle packaged IC's or be used for manual loaded individual packaged IC testing. PA1 2) There are no common interface height and position standards. PA1 3) The prober may be moved between different testers, and must be available for maintenance. Maintenance or adjustment may include the Z, X, and Y stages. In order to make consistent contact, the chuck top must be level with respect to the horizontal XY motion, and the probe card needles must be leveled with respect to the chuck top. The adjustment is usually provided by compression of pads underneath the chuck top or beneath the Z stage by mounting screws or adding thin strips of metal or "shimming". Ring carrier screws mounted at the corners of the ring carrier provide leveling of the probe card. PA1 4) The D/A converters must be available for maintenance. The D/A converters may require adjustment or replacement. PA1 5) The test head must be docked to the prober or make electrical connection between the D/A converters and the probe card. The outside surface of the performance board typically consists of radial connector points. The top surface of the probe card consists of a matching set of connector pads. Electrical contact is achieved using a connector pin ring. The connector pins are miniature compressible connectors that are typically of two-piece construction with an internal spring so multiple contact can be made between two uneven surfaces. A more complex structure with the same operational objective is taught in U.S. Pat. No 5,187,431 where the connector pins are typically held in a plastic ring that is mounted to the top of the ring carrier. PA1 1) The D/A converters can be mounted in the tester minimizing the range of motion required in the D/A converter assemblies and reducing cable lengths or eliminating the need for external cable completely by replacing all cabling by pin connectors. The result will be a significantly higher speed electrical interface. PA1 2) The footprint of the prober and tester combination will be greatly reduced compared to a conventional test head solutions. This will be a particular advantage as the wafer size and probes become larger in the future.
When the performance board is brought into contact with the connector pins, the radial contact points must be aligned with the connector pins. This is achieved using mechanical keys such as a pin that mates with a hole in the test head. This procedure is commonly called docking. The contact points are 1-2 mm in diameter, so this defines the docking accuracy requirement.
There are additional complex interfaces with additional layers and using connectors other than connector pins, as taught in U.S. Pat. No. 5,329,226. However, there remains a general need to accurately dock the test head with the prober. In all cases the test head is provided with the mechanical freedom in all axes to be moved to align to the prober.
A test head can be as large as 91 cm in diameter, and weighs 227 Kg. They contain a large collection of very high performance D/A converters to generate the signals. These test heads have become progressively larger, more unwieldy, needing their own manipulator to move them around and change their orientation. Manipulators are described in U.S. Pat. No. 5,241,870. The test head, manipulator and cables take up more and more costly facility space. Some manufacturers have gone to the extreme of erecting a gantry over the prober in order to move the test head around without using up floor space.
The test frequencies have increased by at least a factor of 10 since the introduction of modern test heads. The tester to D/A converter cables have now become a factor again in test frequency. There is thus a need for a more cost effective solution to providing a very high speed tester interface.
Kensington Laboratories, Inc. of Richmond, Calif. has developed a wafer inspection and defect review station in which a microscope is mounted to inspect a vertically mounted and insertable wafer semiconductor.